Although Apple has not publicly confirmed the need for an update to Lion in order to run iCloud, developers have been testing the service using Mac OS X 10.7.2, an unreleased edition.

Yesterday, Apple issued a GM, or gold master build of 10.7.2 to developers. The term is a label some developers use -- Microsoft calls it "release to manufacturing," or RTM, instead -- for finished software that presumably is ready to send to duplicators and distributors, or in Apple's case, for release to the public.

The notes accompanying the GM of Mac OS X 10.7.2 said there were no known issues with the release, another sign that Apple is close to green-lighting the update.

Apple has scheduled iCloud's roll-out on the same day as the release of iOS 5, and two days before it launches the iPhone 4S.

The company took to a similar timeline in 2008 when it debuted MobileMe, iCloud's predecessor, two days before the on-sale date of that year's smartphone, the iPhone 3G.

MobileMe's launch was rocky at best, a debacle at worst.

The service stumbled even before it got off the ground: The transition from .Mac, which was supposed to take only a few hours, instead dragged on for a full day, raising the ire of users locked out of their accounts. Days later, customers complained about slower-than-expected synchronization, which Apple answered with an apology and a 30-day service extension to all users. Finally, an Apple server went south, taking down the e-mail accounts of about 1% of MobileMe's subscribers. The outage lasted 11 days before service was fully restored.

Then-CEO Steve Jobs later told employees that the MobileMe launch was "simply not up to Apple's standards" and replaced the vice president who had overseen the service with Eddy Cue, the executive who Tuesday demonstrated iCloud at the iPhone 4S unveiling.

Apple has said that iCloud will run only on Mac OS X 10.7, the edition that powered approximately 22% of all Macs that went online during September.

Unlike MobileMe, iCloud is free to all iOS 5 and Lion users. iCloud includes 5GB of free online storage and serves as a music, photo, app, document and other data sync service that keeps multiple devices up-to-date with user-purchased or -created content. Users can purchase additional storage space: $20 annually will cover 10GB of space; $40, for 20GB; and $100 a year, for 50GB.

People who own an iOS 5 device but rely on a Windows PC rather than a Mac must have Windows Vista or Windows 7, Apple said.

Apple will release the 10.7.2 update through its usual Software Update and manual download channels.